Structural specificity in biological interactions involving macromolecules is generally dominated by non-covalent intermolecular forces. While localised biological interactions may be dependent on proton or electron transfer between groups or, in the case of an enzyme action, on the transfer of a functional group between molecules, the orientation and complementarity of the interacting groups usually dictate a much larger structure. The resulting interacting surface is dominated by the non-covalent intermolecular binding forces of the two molecules. Often a biological reaction may require a signal to be transmitted over a distance to a site of action, messages, for example, from periplasmic to cytoplasmic molecules having to traverse the cell membrane through receptor proteins.